It is well-known in the fields of reverse osmosis (including nano-filtration), gas-separation, pervaporation, dialysis, preparative electrolysis, primary and secondary electric storage cells, electrodialysis and reverse dialysis that the resistance to permeation is inversely proportional to the thickness of the active barrier layer. (It also seems to be a fact of life that barrier layers which have high selective permeability also have high resistance per unit thickness to permeation. For example the O.sub.2 /N.sub.2 separation factor for poly (2,6-dimethyl phenylene oxide) is about twice that of poly dimethyl siloxane but the O.sub.2 flux of the latter per unit barrier layer thickness is only about one-fortieth of the former). It is therefore economically important that the selectively permeable barrier layer be as thin as possible in order to reduce overall cost of the apparatus and energy consumed in overcoming resistance to permeation. The desired thin barrier layers naturally have a low burst strength (particularly when they are swollen with some permeants) and require support substrates with very small support spans (e.g. characteristic dimensions of the foramina). Most methods of preparing support substrates for barrier layers unfortunately lead to reduced total porosity when the characteristic dimensions of the foramina are reduced. Furthermore with some exceptions (e.g. track-etched substrates such as those microporous membranes trade-named Nuclepore and Thiele-type ionotropic gel membranes) such support substrates have a fairly wide distribution of the characteristic dimension of the foramina. (Nuclepore microporous membranes have an inherently low porosity and Thiele-type membranes are not commercially available). For example a well-known microporous membrane useful as a substrate is rated at 0.45 micrometers but has about 60 percent of its pores in excess of 0.45 micrometers and about 3 percent of the pores in excess of 2 micrometers. Under stress the barrier layer spanning the latter pores is that most likely to be ruptured. Hence the thickness of the barrier layer must be designed around the few large pores in the substrate leading to lower permeation rates (fluxes) than would otherwise be possible.
In accordance with this invention it has now been discovered that if the porous substrate is treated with an emulsion or latex appropriate under the circumstances before or during the process of affixing or forming the non-porous selectively permeable barrier layer thereon, superior apparatus for separating fluid mixtures into less permeable and more permeable fractions can be easily obtained. Otherwise it would require more rigorous preparation procedures, more careful selection of substrates and/or greater rejection of defective selectively permeable barriers. If the microporous substrate is prepared by phase inversion of a solution of a suitable polymer or mixture of polymers against an aqueous solution which is a poor solvent for such polymers then the emulsion or latex may conveniently be a component of such aqueous solution.
It is therefore an objective of this invention to provide improved apparatus for separating fluid mixtures into less permeable and more permeable fractions, said apparatus comprising at least one selectively permeable barrier which comprises a porous, emulsion treated substrate and a non-porous selectively permeable barrier layer thereon. This and other objectives will be obvious from the following detailed description of the invention.